Field of the Invention
The present invention relates to an image processing technique for correcting an image obtained by reading an original.
Description of the Related Art
In copying machines, multi-function peripherals, and the like there are cases in which a problem known as show-through occurs in a case where an original is read using an image reading apparatus (scanner) installed therein. Show-through is something in which an image on the other surface (a back surface) of the original appears in the read-in image, in a case where a surface of the original (a front surface) is read by an image reading apparatus. Thus, this mainly occurs in cases in which both surfaces (front surface and back surface) of the original that is read by the image reading apparatus have some kind of image printed on them. Show-through tends to occur in cases where a high density image exists on the back surface. Also, this occurs due to the light source, the light amount when reading, and the thickness of the medium of the read-in original (sheet, or the like) (conditions of light penetration). When show-through occurs, an image within the read-in image becomes difficult to see, i.e. the quality of the image deteriorates.
There are cases in which, as a technique for reducing show-through in a read-in image, processing for reducing the density of the image on the whole (strongly applying a so-called background removal function) is used. However, in such cases, not just the show-through but also the density of the image that exists on the front surface of the original is reduced. For this reason, there is a possibility that low-density images will be lost.
In Japanese Patent Laid-Open No. 2012-160883 (patent document 1), for example, a technique is proposed in which a variance value in a fixed range including a pixel of interest is obtained, and when the variance value is not greater than a predetermined reference value, show-through deletion processing is executed. This is processing that focuses on the fact that a low-density portions of the image are expressed in halftone dots, and is something that uses a characteristic that the variance value increases when image data of an area is expressed as halftone dots. The show-through component tends not to be expressed as halftone dots from the perspective of the front surface, and so the variance value is lower. Therefore, dividing up of whether the image is a show-through component or whether the front surface is a low-density portion is performed based on whether or not the variance value is less than or equal to a reference value, and show-through deletion processing is executed only on the show-through components.
However, in magazines and pamphlets comprising mediums (sheets) which have a thin thickness, in a case where an image of a low-density or a medium-density portion (halftone dot region) exists on the front surface, show-through occurs in a way in which the image is overlapped. For a show-through component that occurs in a way in which the low-density or medium-density portion of the image on the front surface is overlapped, the variance value becomes larger, and so this kind of show-through component cannot be appropriately deleted with the above described technique disclosed by patent document 1.